26 research outputs found
Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the
indirectly intersurface coupled topological insulator Bi2-xMnxSe3 films (12 nm
thick) revealed a strong dependence of the rise-time and initial decay-time
constants on photoexcited carrier density and Mn content. In undoped samples (x
= 0), these time constants are exclusively governed by electron-electron and
electron-phonon scattering, respectively, whereas in films with x = 0.013 -
0.27 ultrafast carrier dynamics are completely controlled by photoexcited
electron trapping by ionized Mn2+ acceptors and their dimers. The shortest
decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great
potential of Mn-doped Bi2Se3 films for applications in high-speed
optoelectronic devices. Using Raman spectroscopy exploiting similar laser
photon energy (1.58 eV), we demonstrate that due to indirect intersurface
coupling in the films, the photoexcited electron trapping in the bulk enhances
the electron-phonon interaction strength in Dirac surface states
Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3
Effect of carrier recombination on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) from thin films (6 - 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques
Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in
thickness, of the topological insulator Bi2Se3 revealed a strong dependence of
the carrier relaxation time on the film thickness. For thicker films the
relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution
of the bulk insulating phase dominates over that of the surface metallic phase.
The carrier relaxation time shortens with decreasing film thickness, reaching
values comparable to those of noble metals. This effect may result from the
hybridization of Dirac cone states at the opposite surfaces for the thinnest
films
Resonance-type thickness dependence of optical second harmonic generation in thin-films of the topological insulator Bi2Se3
Optical second harmonic generation (SHG) has been measured for the first time
in reflection from the nanometer-thick films (6 to 40 nm) of the topological
insulator Bi2Se3 using 1.51 eV (820 nm) Ti:Sapphire laser photons and revealed
a strong dependence of the integral SHG intensity on the film thickness. The
integral SHG intensity was determined by area integration of the SHG rotational
anisotropy patterns measured for different input-output light polarization
geometries. A ~100-fold enhancement of the integral SHG intensity with
decreasing film thickness has been suggested to result from the
DC-electric-field-induced SHG (EFISHG) effects. Two sources of dynamically
created DC electric field were proposed: (i) the capacitor-type DC electric
field that gradually increases with decreasing film thickness from 40 to 6 nm
due to a dynamical imbalance of photoexcited long-lived carriers between the
opposite-surface Dirac surface states and (ii) an DC electric field associated
with a nonlinearly excited Dirac plasmon, which is responsible for the resonant
enhancement of the integral SHG intensity for the 10 nm thick film with a
Lorentz-shaped resonance of ~1.6 nm full width at half maximum. Additionally to
the general SHG enhancement trends with decreasing film thickness, a relative
decrease of the out-of-plane contribution with respect to the in-plane
contribution was observed. Using a theoretical treatment of the measured SHG
rotational anisotropy patterns, this effect has been suggested to result from
the joint contributions of the linear and quadratic DC electric field effects
to the EFISHG response
Nonlinear Optical Observation of Coherent Acoustic Dirac Plasmons in Thin-Film Topological Insulators
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient secondharmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac state
Nonlinear Optical Observation of Coherent Acoustic Dirac Plasmons in Thin-Film Topological Insulators
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient secondharmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac state
Effects of Oxygen Modification on the Structural and Magnetic Properties of Highly Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films
La0.7Sr0.3MnO3, a strong semi-metallic ferromagnet having robust spin polarization and magnetic transition temperature (TC) well above 300 K, has attracted significant attention as a possible candidate for a wide range of memory, spintronic, and multifunctional devices. Since varying the oxygen partial pressure during growth is likely to change the structural and other physical functionalities of La0.7Sr0.3MnO3 (LSMO) films, here we report detailed investigations on structure, along with magnetic behavior of LSMO films with same thickness (~30 nm) but synthesized at various oxygen partial pressures: 10, 30, 50, 100, 150, 200 and 250 mTorr. The observation of only (00 l) reflections without any secondary peaks in the XRD patterns confirms the high-quality synthesis of the above-mentioned films. Surface morphology of the films reveals that these films are very smooth with low roughness, the thin films synthesized at 150 mTorr having the lowest average roughness. The increasing of magnetic TC and sharpness of the magnetic phase transitions with increasing oxygen growth pressure suggests that by decreasing the oxygen growth pressure leads to oxygen deficiencies in grown films which induce oxygen inhomogeneity. Thin films grown at 150 mTorr exhibits the highest magnetization with TC = 340 K as these thin films possess the lowest roughness and might exhibit lowest oxygen vacancies and defects. Interpretation and significance of these results in the 30 nm LSMO thin films prepared at different oxygen growth pressures are also presented, along with the existence and growth pressure dependence of negative remanent magnetization (NRM) of the above-mentioned thin films
Nanoscale control of exchange bias with BiFeO3 thin films
We demonstrate a direct correlation between the domain structure of
multiferroic BiFeO3 thin films and exchange bias of Co0.9Fe0.1/BiFeO3
heterostructures. Two distinct types of interactions, an enhancement of the
coercive field (exchange enhancement) and an enhancement of the coercive field
combined with large shifts of the hysteresis loop (exchange bias), have been
observed in these heterostructures, which depend directly on the type and
crystallography of the nanoscale (2 nm) domain walls in the BiFeO3 film. We
show that the magnitude of the exchange bias interaction scales with the length
of 109 degree ferroelectric domain walls in the BiFeO3 thin films which have
been probed via piezoresponse force microscopy and x-ray magnetic circular
dichroism.Comment: Accepted to Nano Letters May 200